1. Field of Invention
The invention relates to a flow cell for liquid chromatography, fluorometry and the like wherein optical radiation is directed through a sample within the flow cell and viewed by an optical detector for light absorption or fluorescence.
A flow cell is a device for use in a fluid analysis system such as chromatograph or a fluorometer. Commonly, the cross-sectional dimension of the fluid path is one millimeter or less in order to accommodate the extremely small volumes of liquid which are to be analyzed. In fact, it is desirable to achieve liquid homogeneity with less than one-half milliliter volume. It is also desirable to maximize the volume of the viewing area in order to enhance the yield of detected signals for such small volumes. Certain fixed geometries for flow cells severely limit shape and size. For example, if it is desired to view a sample transverse to the normal flow path, it is necessary to provide a viewing area in a flow cell which is of substantially larger diameter than the diameters of the respective inlets and outlets. The cross-sectional areas of the inlets and outlets are of course restricted to limit the volume of material resident in inlet and outlet conduits. One common geometry includes a cylindrical chamber having an inlet and outlet with inner diameter substantially smaller than the inner diameter of the chamber.
A major problem in the use of flow cells is the phenomenon of carryover. Carryover is the residue of a preceding portion remaining in a flow cell at the time of the next detection cycle which causes errors in signal output. In order to minimize carryover, a flow cell is typically flushed with a cleaning solution between samples. A sharp boundary transition between the inlet orifice and the flow cell viewing chamber gives rise to eddies in fluids moving through the chamber such that the chamber is not adequately clean when flushed with a cleaning solution. Even copius amounts of cleaning solution may be inadequate to eliminate carryover. One partial resolution has been the provision of a countersunk inlet orifice and a deflector in the inlet to create turbulence in the inflowing cleaning solution. These expedients have been only partially successful and have imposed practical limitations on the maximum diameter of the flow cell.
Another problem has been inordinately long thermal equilibration times. The flow cell itself has in the past been used as an equilibration chamber wherein a portion of the walls of the chamber includes a heating element. The minimum achievable time of equilibration is limited because only a portion of the fluid is adjacent the heating element in the flow cell, since a substantial portion of the flow cell must be devoted to window space for a nonthermally conductive window. An alternative technique for achieving more rapid equilibration is desirable to decrease equilibration times.
There is a need for a flow cell having a size and geometry which maximizes signal yield and minimizes carryover. There is also needed a flow cell which requires a minimal amount of cleaning solution for flushing the viewing chamber and for clearing the inlet and outlet conduits.
2. Brief Description of the Prior Art
The following patents are of interest for showing flow cell designs:
Gorgone et al., U.S. Pat. No. 3,646,313 for TEMPERATURE CONTROLLED FLOW CELL;
Emmel et al., U.S. Pat. No. 3,647,304 for MICROVOLUME FLOW CELL;
Bellinger et al., U.S. Pat. No. 3,740,158 for FLOW CELL;
Aday, Jr., U.S. Pat. No. 3,822,947 for FLUID SAMPLE FLOW CELL;
Heiss, U.S. Pat. No. 3,917,404 for FLUOROMETER ASSEMBLY INCLUDING A FLOW CELL;
Munk, U.S. Pat. No. 4,006,990 for CONVERGENT LIGHT ILLUMINATED FLOW CELL FOR LIQUID CHROMATOGRAPHY;
Zdrodowski, U.S. Pat. No. 4,008,397 for FLUOROMETER FLOW CELL; and
Shigetomi, U.S. Pat. No. 4,141,954 for REACTION TUBE ASSEMBLY FOR AUTOMATIC ANALYZER.